The invention relates to modems which convert digital signal levels (mark, and space signals) on a "data" lead to corresponding sine wave signals on a telephone line, and vice versa. More particularly, the invention relates to a frequency shift key modulator, which is the transmitter portion of such a modem, and provides sine wave signals of different frequencies on an output which correspond to the mark (1) and space (0) signals on a "send data" input lead.
Previous frequency shift key modulators (the transmitting portions of modems) have generally been of the analog type and have generally included a voltage controlled multivibrator which first produces a rectangular frequency shift key pulse train and filters for then filtering out the higher harmonics from the rectangular pulse train. These modems have been quite large and expensive; and, in addition, they are not voltage compatible with the new large scale integration logic technologies.